Nanoparticle probes for tip-enhanced spectroscopy

Abstract

For exploring chemical and physical properties of nanoscale objects, chemical information of single molecule and optical images with high spatial resolution is imperative. In this regard, tip-enhanced near-field optical microscopy (TENOM) is an evolving field having extensive potential. Because it provides higher spatial resolution as well as the electric field enhancement at the tip-end. This means, in other words, plasmonic enhancement and spatial confinement within the tip are critically connected to the enhanced optical signal. Therefore, we explored enhanced optical signals with various kinds of the probes to find the optimal plasmonic properties of the probes. To apply the TENOM, we use a confocal microscopy with a radially polarized light at 532 nm wavelength and an atomic force microscopy (AFM) fitted with the tip. To check the validation of our experimental setup, we first examined the radial polarizer which generates the polarization component of a laser at 532 nm parallel to the tip (z-aixs). By getting fluorescence images of nile red (in polystyrene beads with a diameter 20 nm) in both radial and azimuthal mode, we were convinced that polarization of laser was parallel to the z-axis in the focal plane. And then, experiments for tip-enhanced Raman scattering (TERS) and fluorescence (TEF) were conducted with two types of special fabricated probes which were synthesized in a similar way. In order to guarantee the our setup is available for the TERS and TEF measurements, we used a nanoparticle - a Au core/Ag shell nanoparticle (Au@AgNP) with a controlled thickness of shell - attached probe. For the synthesis of Au@AgNP probe, a seed Au nanoparticle (NP) is picked up by DNA-DNA interaction and enhanced with a silver solution were employed. For the TERS measurement, the self-assembled biphenyl-4-thiol (BPT) monolayer on Au film as the sample was employed with the Au@AgNP (size: 20-90 nm) probe. We find that such tips show high plasmonic field enhancement and this reveals clearly as size of NPs increases. For a further study, we developed a scanning probe with a metallic nanostar (a core diameter of ~70 nm and spike lengths between 50 nm and 80 nm). The approach involves AuNP pickup process and growth of spikes through reduction of Au3+ with ascorbic acid in the presence of Ag+. The nanostar probe makes the narrow enhancement field on account of its sharp edge. We find that nearly all of the tips show the local field enhancement up to 100-fold with an improved optical resolution below 100 nm for TERS and TEF was confirmed. These were evaluated with DiI dye for TEF and crystal violet for TERS. The current probe, however, shows large tip-to-tip variability, which may arise from the uncontrolled orientation of the apexes of the spike with respect to the sample surface, which calls for the further fabrication improvement. The result overall supports a new fabrication approach for the probe that is effective for tip-enhanced spectroscopy.